Varactors may be diodes that act as voltage-controlled capacitors. As a control voltage across a layer of the varactor varies, the capacitance of the varactor may also vary. This variance may be called “tuning.” Generally, semiconductor varactors may have a wider tuning range (i.e. capacitance variance) and lower control voltage requirements than dielectric varactors realized on materials such as barium strontium titanate (BST). However, the semiconductor varactors may typically achieve a lower capacitance per unit area than a dielectric varactor, thereby requiring a larger die area to implement a given capacitance.
Generally, a varactor may be considered a two-port device, i.e. having two input terminals and two output terminals. As such, varactors may be prone to self-modulation distortion resulting from applied radio frequency (RF) voltages. This self-modulation distortion may introduce nonlinearity into a circuit using the varactors. To reduce this nonlinearity to acceptable levels, a number of individual varactors may be coupled in series to divide the RF voltage across them. If the number of varactors in the series is n, then the die area on the circuit board required to realize a desired net capacitance may be increased by a factor of n2 if the varactors are co-planar to one another. If a relatively large number of varactors is used, then this circuit may make the required die area prohibitively large for use in modern devices.